MORE COMPLICATED FORMS. 



rl S 



sist of — (i) a flat ectoderm ; (2) a mesogloea containing 

 triradiate calcareous spicules, phagocytes, and reproductive 

 elements ; and (3) an endoderm lining the central cavity, 

 and composed of collared flagellate cells, almost exactly like 

 some of the monad Infusorians. This simple sponge is not 

 much above the gastrula level ; it agrees generally with a 

 simple Ccelenterate, such as Hydra, but differs from it in 

 the absence of tentacles and stinging cells, and in the 

 greater development of the mesogloea. 



More complicated forms.— But a description of a simple 

 sponge like Ascetta conveys little 

 idea of the structure of a complex 

 form such as the bath -sponge 

 (Euspongia). Let us consider the 

 origin of complications. 



(a) Sponges — long regarded as 

 plants — are plant - like in being 

 sedentary and passive. They seem 

 also to feed easily and well. Like 

 plants, they form buds, the outcome 

 of surplus nourishment. These 

 buds, like the suckers of a rose- 

 bush, often acquire some apparent 

 independence, and the sponge looks 

 like many vases, not like one. More- 

 over, as they grow these buds may 

 fuse, like the branches of a tree tied 

 closely together. Thus the structure 

 becomes more intricate. 



(b) In the simple sponge the 

 gastric cavity of the vase is com- 

 pletely lined by the collared endoderm cells {Ascon 

 type). But the endoderm may grow out into radial 

 chambers, and the walls of these may also be folded into 

 side aisles (Sycon type). The outgrowing of the endoderm 

 into the mesogloea may be continued even further, and the 

 cells may become pavement-like, except in the minute 

 flagellate chambers, where the characteristic collared type 

 is retained {Leucon type). (See Fig. 50.) 



[Speculatively, it may be suggested that the characteristic 

 folding or outgrowth of the endoderm is necessitated by the 



Fig. 49.— Section of a 

 sponge. — After F. E. 

 Schulze. 



Showing inhalant canals, 

 flagellate chambers, a 

 gastrula forming in the 

 mesoglcea, etc. 



